It's a trick I stole from ext2, and simplified. In that filesystem there are three bitsets: one for reading, one for writing, one for fsck. If you don't understand a bit you can't do that action.
For most protocols there's only reading and writing, so you can use odd bits to mean "backwards compatible features, you can read even if you don't understand" and even for "stop, we broke compat".
That's a good idea for filesystems. But OpenTimestamps Proofs aren't really "written to". They're created, and then later validated. Also, being cryptographic proofs, my philosophy is the validator should almost always understand them 100%, or not at all, to avoid any false proofs.
That's also why I picked a binary encoding: it's difficult to parse an OTS proof incorrectly. An incorrect implementation will almost always fail to parse the proof at all, with a clear error, rather than silently parse the proof incorrectly.
